EXCHANGE 


8061  '12  WT  ' 


The  Quantitative  Determination  of 
Chromium 


THESIS 

Presented  to  the  Faculty  of  the  Department  of  Philosophy 
of  the  University  of  Pennsylvania,  in  Partial  Fulfil- 
ment of  the  Requirements  for  the  Degree 
of  Doctor  of  Philosophy 


BY 
THOMAS  RUSH  ALEXANDER,  JR. 

PHILADELPHIA,   PA. 
1915 


The  Quantitative  Determination  of 
Chromium 


THESIS 

Presented  to  the  Faculty  of  the  Department  of  Philosophy 
of  the  University  of  Pennsylvania,  in  Partial  Fulfil- 
ment of  the  Requirements  for  the  Degree 
of  Doctor  of  Philosophy 


BY 
THOMAS  RUSH  ALEXANDER,  JR. 

PHILADELPHIA,   PA. 
1915 


WASHINGTON,  PA. 

OBSERVER  JOB  ROOMS 

1917 


The  writer  takes  this  opportunity  to  express  his 
indebtedness  to  the  suggestions,  advice  and  unfail- 
ing kindness  of  Dr.  Edgar  F.  Smith  for  any  merit 
which  this  investigation,  may  possess. 


INTRODUCTION 

Careful  analysts  know  that  in  the  gravimetric  determination 
of  chromium  in  the  form  of  sesquioxide  the  latter  is  almost 
invariably  contaminated  with  alkali  chromate.-  To  correct  this 
error  it  has  been  customary  after  the  ignition  of  the  chromium 
sesquioxide  to  digest  it  with  water.  In  the  presence  of  alkali 
chromate  the  water  is  colored  yellow  and  it  has  been  customary 
to  treat  the  sesquioxide  with  water  until  it  ceases  to  impart  a 
color.  The  colored  extracts  or  filtrates  may  be  reduced  to  a 
small  volume  in  a  glass  vessel  and  mercurous  nitrate  introduced. 
The  resulting  mercurous  chromate,  after  filtration,  is  ignited  in 
a  weighed  porcelaine  crucible.  There  remains  only  chromium 
sesquioxide.  The  alkali  chromate  so  often  present  in  ignited 
sesquioxide  of  chromium  is  never  negligible.  The  error  intro- 
duced by  it  varies  in  magnitude  and  should  always  be  taken  into 
consideration.  The  purpose  of  the  present  study  has  been  to 
determine  under  what  conditions,  if  any,  chromium  sesquioxide 
may  be  obtained  free  from  alkali. 


444342 


EXPERIMENTS 

A  solution  of  chrome  alum  of  known  chromium  content  was 
prepared.  Fifty  c.c.  were  treated  with  ammonia  gas  and  after 
the  resulting  chromium  hydroxide  had  partially  dissolved  in  the 
ammonium  hydroxide,  heat  was  applied  until  complete  precipi- 
tation took  place.  The  resulting  precipitate  was  washed  by 
decantation  with  water,  approximately  fifty  c.c.  of  water  being 
used  for  each  washing;  after  which  the  washing  was  continued 
on  the  filter  with  the  aid  of  suction.  On  evaporating  the  wash 
water  chromium  was  not  found  in  the  residue  nor  was  it  possible 
to  get  a  greater  purity  of  the  oxide  upon  using  more  than  five 
hundred  c.c.  of  water  for  washing  purposes.  The  precipitation 
of  chromium  hydroxide  took  place  first  in  beakers  of  Jena  glass ; 
later  dishes  of  fused  silica  and  finally  platinum  dishes  were  used. 
The  chromium  hydroxide  was  always  ignited  in  platinum  cruci- 
bles. Upon  washing  it  after  ignition,  with  water,  the  latter  in- 
variably took  on  a  yellow  color.  Some  of  the  results  obtained 
were  as  follows : 

CHROME  ALUM  SOLUTION 


Chromic  Oxide           Jena  Glass                Quartz                  Platinum 

taken                       found                    found                     found 

0.2349  gms.            0.2375  gms.            0.2362  gms.            0.2350  gms. 

0.2349 

0.2384 

0.2358 

0.2358 

0.2349 

0.2372 

0.2348 

0.2353 

0.2349 

0.2375 

0.2356 

0.2340 

0.2349 

0.2376 

0.2362 

0.2367 

0.2349 

0.2370 

0.2359 

0.2365 

0.2349 

0.2369 

0.2363 

0.2360 

0.2349 

0.2372 

0.2360 

0.2369 

Average                   0.2374      "              0.2359 

0.2358      " 

%   alkali                  1.06          "              0.43 

0.38 

A  second  series  of  experiments  wras  instituted  with  chromium 
sulphate.  From  appended  results  it  will  be  seen  that  chromium 
determinations  were  more  •  accurate,  but  such  conditions  do  not 
ordinarily  obtain  in  the  course  of  an  analysis.  Further,  the 
determinations  indicate  that  the  presence  of  even  small  quantities 
of  alkali  salts  in  the  solution  from  which  the  chromium  is  pre- 
cipitated render  the  method  unreliable.  Results : 


CHROMIUM  SULPHATE  SOLUTION 


Chromic  Oxide           Jena  Glass                Quartz 

Platinum 

taken                      found                    found 

found 

0.2392  gms.            0.2422  gms.            0.2400  gms.            0.2400  gms. 

0.2392 

0.2420 

0.2380 

0.2398 

0.2392 

0.2400      ' 

0.2426      ' 

0.2413 

0.2392 

0.2401 

0.2413      ' 

0.2392 

0.2392 

0.2419 

0.2405 

0.2408 

0.2392 

0.2417 

0.2401      ' 

0.2395 

0.2392 

0.2406 

0.2396      ' 

0.2395 

0.2392 

0.2400 

0.2397      ' 

0.2399 

Average                   0.2411      "               0.2402      ' 

0.2400 

%  alkali                   0.84          "              0.41 

0.33 

The  method  suggested  long  ago  by  Berzelius  for  the  gravi- 
metric estimation  of  chromium  is  familiar  to  all  analysts.  Modi- 
fications of  it  have  been  suggested  from  time  to  time,  but  these 
too  are  so  familiar  that  a  detailed  description  of  them  is  not 
necessary.  On  dissolving  chronium  hydroxide  in  an  excess  of 
fixed  alkali  and  oxidizing  the  solution  with  hydrogen  peroxide 
subsequently  precipitating  mercurous  chromate  and  igniting  it 
to  chromium  sesquioxide,  satisfactory  and  concordant  results 
were  not  obtained  because  it  was  impossible  to  completely  elim- 
inate the  alkali  salts  from  the  mercurous  chromate  even  by  pro- 
longed washing  with  water  by  decantation.  The  sodium  or  pot- 
assium salts  in  the  solution  were  responsible  for  the  error,  hence 
an  effort  to  eliminate  them  was  made  along  the  following  lines. 
Into  a  solution  of  chrome  alum  of  known  chromium  content  was 
brought  ammonia  gas  until  a  portion  of  the  precipitated  chrom- 
ium hydroxide  had  redissolved.  To  this  solution  hydrogen 
peroxide  was  added  and  at  the  end  of  half  an  hour  the  oxidation 
of  chromium  oxide  to  chromic  acid  was  apparently  complete.  It 
was  discovered  later  on  that  if  sufficient  time  was  allowed  for  the 
hydrogen  peroxide  to  act  in  the  cold,  before  applying  any  heat, 
the  oxidation  of  the  chromium  to  chromic  acid  was  always  com- 
plete. When  heat  was  applied  too  soon  a  portion  of  the  chrom- 
ium hydroxide  almost  invariably  remained  unoxidized. 

To  a  solution  such  as  has  just  been  described  mercurous 
nitrate  was  added.  The  resulting  mercurous  chromate  was  filter- 
ed out  and  washed  with  a  dilute  solution  of  mercurous  nitrate 
The  chromium  sesquioxide  remaining  after  the  ingnition  of  the 
mercurous  chromate  in  a  platinum  crucible  contained  chromate, 


so  that  the  results  were  always  high.  It  seemed  possible  that 
more  complete  washing  of  the  mercurous  chromate  might  reduce 
the  error,  hence  after  executing  the  oxidation  as  outlined  in  the 
preceding  lines,  the  liquid  was  diluted  to  about  four  hundred 
c.c.  and  heated  to  boiling ;  mercurous  nitrate  was  introduced  and 
the  resulting  mercurous  chromate  was  allowed  to  settle,  the  super- 
natant liquid  being  decanted  through  a  filter.  The  residue  of 
mercurous  chromate  in  the  breaker  was  boiled  for  a  minute  with 
fifty  c.c.  of  water,  allowed  to  cool,  and  a  few  drops  of  the  mercu- 
rous nitrate  solution  introduced.  After  the  precipitate  had  set 
tied  the  liquid  was  decanted  and  the  washing  repeated.  Naturally 
small  quantities  of  mercurous  chromate  were  transferred  to  the 
filter  in  each  decantation  and  the  operation  was  continued  until 
nearly  the  whole  of  the  precipitate  had  been  so  transferred. 

The  last  traces  of  the  precipitate  were  assembled  from  the 
beaker  and  the  washing  continued  on  the  filter  with  dilute  mer- 
curous nitrate,  suction  being  applied  after  each  washing.  The 
results  obtained  by  this  procedure  are  as  follows : 


Chromic  Oxide 
taken 

0.2948  gins. 

0.2948 

0.2948 

0.2948 

0.2948 

0.2948 

0.2948 

0.2948 

0.2948 

0.2948 


Chromic  Oxide 

found 

0.2931  gins. 
0.2939 
0.2939 
0.2950 
0.2947 
0.2948 
0.2945 
0.2949 
0.2939 
0.2951 


Average       0.2943 
Average  Error  — 0.16% 

The  ease  with  which  a  cromium  salt  may  be  converted  into  a 
chromate  by  the  method  just  outlined  suggested  that  possibly  it 
might  also  prove  of  value  in  the  separation  of  chromium  from 
other  metals. 

SEPARATION  OF  CHROMIUM  FROM  IRON 

To  portions  of  a  solution  of  chrome  alum  slightly  acidulated 
with  sulphuric  acid,  various  amounts  of  ferrous  ammonium  sul- 
phate were  added  and  the  treatment  with  ammonia  and  hydrogen 

6 


peroxide  carried  out  as  above.  After  the  excess  of  ammonia  and 
hydrogen  peroxide  had  been  removed  the  precipitated  ferric 
hydroxide  was  filtered  and  washed,  then  transferred  to  a  breaker 
and  boiling  water  added.  The  supernatant  liquid  was  yellow,  but 
after  filtering  and  washing  again  it  was  colorless.  Examination 
showed  that  the  ferric  hydroxide  still  contained  small  amounts  of 
chromium  so  the  experiment  was  repeated  with  this  modification : 
after  the  first  treatment  with  ammonia  and  hydrogen  peroxide 
the  precipitate  was  allowed  to  settle  and  the  supernatant  liquid 
decanted,  care  being  taken  to  keep  as  much  of  the  precipitate  as 
possible  in  the  beaker.  To  the  precipitate  water  was  added  and 
the  treatment  with  ammonia  and  hydrogen  peroxide  repeated. 
The  ferric  hydroxide  so  produced  was  found  to  be  free  from 
chromium  except  in  cases  where  relatively  large  amounts  of  iron 
were  present.  Under  these  conditions  it  is  best  to  repeat  the 
addition  of  ammonia  and  hydrogen  peroxide  several  times.  The 
combined  filtrates  and  wash  water  were  boiled  to  expel  the  excess 
of  ammonia  and  hydrogen  peroxide.  During  this  operation  con- 
siderable ferric  hydroxide  separted  and  was  removed  by  filtra- 
tion. The  chromium  was  determined  by  precipitation  with  mer- 
curous  nitrate  as  already  outlined.  Tests  failed  to  show  the  pres- 
ence of  iron  in  the  filtrate.  The  results  were  as  follows : 

Mohr's  Salt  Chromic  Oxide 

Taken  Taken  Found 

0.0291  gms.  0.1873  gms.  0.1868  gms. 


0.1054 
0.1007 
0.2901 
0.8063 
0.3112 
8.0747 


0.1873      "  0.1867 


0.1873 
0.1873 
0.1873 
0.1873 
0.1873 


0.1870 
0.1869 
0.1877 
0.1872 
0.1868 


Average  Error  — 0.16% 

SEPARATION  OF  CHROMIUM  FROM  MANGANESE 

It  seems  probable  that  the  separation  of  chromium  from 
manganese  could  be  accomplished  in  the  same  way  as  the  separa- 
tion of  chromium  from  iron.  A  standard  solution  of  potassium 
permanaganate  was  made,  and  portions  of  twenty-five  c.c.  were 
reduced  with  hydrogen  peroxide  after  accidulation  with  sul- 
phuric acid.  A  standard  solution  of  potassium  dichromate  was 
reduced  in  the  same  way,  and  after  boiling  each  solution  for  a 

7 


few  minutes  to  remove  the  excess  of  peroxide  they  were  poured 
into  the  same  beaker  and  treated  with  ammonia  followed  by 
the  addition  of  hydrogen  peroxide.  It  was  found  necessary  to 
treat  the  precipitate  so  produced  with  ammonia  and  hydrogen 
peroxide  several  times  in  order  to  oxidize  the  last  traces  of 
chromium.  In  all  probability  there  is  a  small  quantity  of  man- 
ganous  chromate  formed  which  is  only  slowly  decomposed  by 
ammoniacal  hydrogen  peroxide.  Complete  solution  of  the  chrom- 
ium was  obtained  in  every  case  after  three  treatments  with  am- 
monia and  hydrogen  peroxide,  the  liquid  being  well  boiled  after 
each  addition  of  hydrogen  peroxide  and  the  volume  kept  at  about 
one  hundred  c.c.  The  separation  of  the  chromium  and  the  man- 
ganese was  complete,  as  is  shown  by  the  following  results.  The 
chromium  was  precipitated  as  mercurous  chromate  and  weighed 
as  chromium  sesquioxide : 

Potassium  Permanganate  Chromic  Oxide 

Taken  Taken  Found 

0.0183  gms.  0.2007  gins.  0.1999  gins. 

0.0183      "  0.2007      "  0.2007      " 

0.0183      "  0.2007      "  0.2009      " 

0.0183      "  0.2007      "  0.2011      " 

0.0183      "  0.2007      "  0.2004      " 

Average  Error  — 0.04% 

The  presence  of  a  slight  yellow  color  in  the  nitrates  obtained 
after  extracting  the  ignited  chromium  sesquioxide  with  water  is 
not  always  indicative  of  the  presence  of  soluble  chromate,  but 
may  be  due  to  the  presence  of  very  finely  divided  sesquioxide  in 
suspension.  During  the  course  of  this  investigation  it  was  fre- 
quently noticed  that  although  filtrates  so  obtained  had  a  yellow 
color  they  did  not  respond  to  the  tests  for  chromates.  By  shaking 
the  liquid  with  finely  divided  asbestos  and  filtering  through  a 
thick  layer  of  asbestos  in  a  Cooch  filter  the  suspended  sesquioxide 
may  be  completely  removed. 

VOLUMETRIC  DETERMINATION  OF  CHROMIUM 

Burrion  and  Seneschal*  have  recommended  the  following 
method  for  the  volumetric  estimation  of  chromium  in  chromic 
salts : 


^Compt.  rend.,  157, 1528. 


The  chromic  salt  solution  after  the  addition  of  an  excess  of 
alkali  is  treated  with  an  excess  of  hydrogen  peroxide  to  convert 
the  chromite  to  chromate.  After  boiling  to  destroy  the  excess  of 
hydrogen  peroxide,  the  solution  is  acidulated  with  sulphuric  acid 
and  the  chromate  reduced  with  a  measured  excess  of  ferrous 
ammonium  sulphate  and  the  excess  of  ferrous  salt  titrated  with 
a  standard  potassium  permanganate  solution.  Some  determina- 
tions carried  out  as  recommended  did  not  give  entirely  satisfac- 
tory results.  The  values  obtained  were  low  and  the  reason  seemed 
to  be  that  the  boiling  did  not  entirely  destroy  the  excess  of  hydro- 
gen peroxide  in  the  alkaline  solution.  Alkaline  chromate  solu- 
tions showed  the  presence  of  hydrogen  peroxide  even  after  long 
continued  boiling,  for  after  acidulation  under  a  layer  of  ether 
the  blue  perchromate  color  could  be  detected.  In  an  attempt  to 
complete  the  decomposition  of  the  excess  of  hydrogen  peroxide  a 
piece  of  platinum  covered  with  platinum  black  was  introduced 
into  the  solution,  but  this  did  not  serve  to  remove  the  last  traces 
of  peroxide  oxygen  and  the  results  were  correspondingly  low. 

Hydrogen  peroxide  in  contact  with  silver  oxide  produces 
metallic  silver  and  oxygen  is  evolved,  so  a  few  drops  of  silver 
sulphate  solution  were  added  to  the  strongly  alkaline  chromate 
solution  and  the  chromium  determination  carried  out  as  recom- 
mended by  Burrion.*  The  results  obtained  were  still  a  trifle  low ; 
this  may  be  due  to  the  presence  of  small  amounts  of  metallic 
silver  suspended  in  the  solution,  producing  gradual  reduction  of 
the  potassium  permanganate.  This  explanation  seems  to  be  all 
the  more  probable  in  view  of  the  fact  that  the  end  point  of  the 
titration  with  potassium  permanganate  was  rather  indefinite. 
The  purple  color  was  slowly  discharged,  and  if  an  amount  of 
permanganate  sufficient  to  produce  a  color  which  remained  for 
one  minute  was  used,  the  quantity  of  chromium  indicated  was 
low.  The  same  titration,  carried  out  with  a  potassium  dichromate 
solution  to  which  no  hydrogen  peroxide  had  been  added,  gave 
good  values  for  chromium  and  a  sharp  end  point.  It  is  possible, 
after  becoming  accustomed  to  the  use  of  this  method  to  select  an 
end  point  which  will  give  quite  satisfactory  results. 


*Loc.  cit. 


The  substitution  of  ammonia  for  fixed  alkaline  hydroxides 
proved  to  be  a  great  benefit,  owing  to  the  ease  with  which  the 
last  traces  of  active  oxygen  could  be  expelled  from  the  solution. 

The  plan  adopted  was  the  same  as  the  one  used  for  convert- 
ing the  chromic  salt  to  chromate  preparatory  to  precipitation 
with  mercurous  nitrate.  After  saturation  with  ammonia  and 
addition  of  hydrogen  peroxide  the  reaction  was  allowed  to  con- 
tinue in  the  cold  until  complete.  After  boiling,  the  chromate 
solution  was  acidified  and  a  weighed  coiantity  of  ferrous  am- 
monium sulphate  added.  The  solution  was  diluted  to  a  volume 
of  nearly  one  liter  and  the  excess  of  ferrous  salt  titrated  with 
potassium  permanganate.  The  end  point  of  the  titration  is  quite 
distinct,  the  only  percaution  necessary  being  that  a  large  volume 
of  water  be  added,  as  recommended  by  Burrion.  The  results 
given  below  were  obtained  by  using  a  solution  of  potassium 
dichromate  which  had  been  standardized  against  iron  wire  and 
ferrous  ammonium  sulphate  in  the  usual  way.  Portions  of 
twenty-five  c.c.  were  taken  from  a  burette  and  reduced  after  acid- 
ulation,  with  hydrogen  peroxide ;  the  excess  of  oxygen  being 
removed  by  boiling  and  the  chromium  redetermined  as  indicated 
above. 


CHROMIC  OXIDE 


Takne 

0.2007  gms. 

0.2007 

0.2007 

0.2007 

0.2007 

0.2007 

0.2007 

0.2007 

0.2007 


Found 
0.2009  gms. 
0.2013 
0.2010 
0.2002 
0.2010 
0.2006 
0.2010 
0.2008 
0.2006 


Average  Error  +0.04% 

Finally  portions  of  twenty-five  c.c.  of  the  standard  potassium 
dichromate  solution  were  reduced  with  ferrous  ammonium  sul- 
phate and  the  excess  of  ferrous  iron  determined  by  titration  with 
potassium  permanganate.  The  solution  was  then  subjected  to  the 
action  of  ammonia  and  hydrogen  peroxide,  the  precipitated  iron 
and  manganese  removed  and  the  chromium  redetermined  in  the 
filtrate  by  precipitation  with  mercurous  nitrate  and  ignition  to 
chromium  trioxide. 


10 


At  least  three  treatments  with  ammoniacal  hydrogen  per- 
oxide were  required  to  completely  remove  the  chromium.  The 
results  obtained  were : 

CHROMIC  OXIDE 

Taken  Found  by  Titration  Found  by  Precipitation 

0.2007  gms.  0.2008  gms.  0.2005  gms. 

0.2007      "  0.2010      "  0.2004      " 

0.2007      "  0.2011      "  0.2009      " 

Average  Error  +0.14%  — 0.04% 

SUMMARY 

1.  In   order   to   convert   chromic   salts   qualitatively    into 
chromic  oxide,  they  should  first  be  converted  into  mercurous 
chromate  and  then  ignited.    The  quantity  of  alkali  salts  present 
should  be  small.    To  insure  this  condition  ammoniacal  hydrogen 
peroxide  can  be  employed  as  an  oxidizing  agent.     The  precipi- 
tation of  mercurous  chromate  should  take  place  from  a  dilute 
solution. 

2.  Chromium  may  be  readily  separated  from  iron  and  man- 
ganese by  means  of  ammoniacal  hydrogen  peroxide. 

3.  Ammoniacal  hydrogen  peroxide  may  be  used  to  convert 
chromium  salts  to  chromates  preliminary  to  the  volumetric  deter- 
mination of  chromium  with  a  ferrous  salt  and  potassium  perman- 
ganate. 


11 


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